1. Field of the Invention
The present invention relates to an optical disk apparatus that, for example, converges a luminous flux emitted from a light source on an information recording surface through a transparent substrate of an optical disk, and performs information recording and reproduction on the information recording surface.
2. Related Art of the Invention
Light converged on the information recording surface of an optical disk is converged into the smallest light spot when the phase is uniform. However, in actuality, aberrations exist in optical systems that condense a luminous flux emitted from a light source on the information recording surface of an optical disk. Because of this, the phase becomes nonuniform, so that the luminous flux is converged into a light spot slightly larger than a so-called diffraction-limited light spot formed when there is no aberration. Aberrations in optical systems that are permissible in an optical apparatus are generally known as the Marechal criterion, and the value thereof is, when the wavelength of the light is λ, substantially 0.07 λ in RMS (root-mean-square) value.
Since the intensity of a light spot exhibits a Gaussian distribution where the intensity is maximum at the center and rapidly decreases with distance from the center, the energy supplied to the recording film of the disk during recording is concentrated on the center of the spot.
At the same light source output, when the spot diameter increases due to aberrations in the optical system, the energy at the center of the spot is lower than that when there is no aberration, so that the energy supplied to the recording film is lower. This degrades recording characteristics.
The aberrations in optical systems include aberrations of optical parts, aberrations due to residual adjustment errors of optical systems, aberrations due to tilts of optical disks, base material thickness errors of optical disks and variations in refractive index and aberrations due to control errors such as defocus.
Of these aberrations, aberrations of optical parts and aberrations due to residual adjustment errors of optical systems are specific to apparatuses. Therefore, the light source outputs can be adjusted in advance by initial adjustment of optical disks. Of other energy decreasing factors, for the factors that do not vary with disk rotation (specifically, factors dependent on the composition of the recording film of the optical disk), the light source output set value that yields the best recording quality can be obtained by so-called recording power learning to perform trial recording while changing the light source output in several steps.
This prior art is effective against the energy varying factors that do not vary with disk rotation and tries to provide a practicable optical disk apparatus without limiting the aberrations in optical systems to strict specifications that are difficult to realize.
However, when there is a factor that varies with disk rotation such as a local variation in disk tilt or a local variation in base material thickness and the energy density reduction cannot be compensated for by the recording power learning or a tilt control to tilt the entire optical system so that the optical axis is vertical to the disk, reduction in recording signal quality cannot be avoided.
Specifically, the light source wavelength has been decreasing and the density has been increasing with increasing NA (numerical aperture) particularly in recent years, and with this, spherical aberration due to a variation in disk base material thickness increases.
For example, when the wavelength and the NA are 650 nm and 0.6 which are used for DVDs, respectively, the spherical aberration caused by a base material thickness error of 10 μm is only 0.01 λ; however, when the wavelength and the NA are 405 nm and 0.85, respectively, the spherical aberration is 0.10 λ which exceeds the Marechal criterion and is as large as ten times the spherical aberration caused in the case of DVDs.
As is apparent from this, in the latter case, to suppress the spherical aberration to approximately 0.03 λ that does not disturb recording and reproduction, a base material thickness error of only 3 μm is permissible. However, a base material thickness variation of this extent can occur within one track of a disk. Particularly, on inner and outer tracks of a disk, the variation can be larger. In such a case, a recording energy variation that cannot be compensated for by the recording power learning occurs due to a variation in spherical aberration, so that deterioration of recording signal quality cannot be avoided.
For factors that vary moderately, for example, a warp of the disk due to a temperature increase in the optical disk apparatus, it is necessary to perform the recording power learning over again as required, and when image data continuing for a long time is recorded, learning cannot always be performed at an appropriate time. In a case where learning is performed when recording is started, there is a wait of the time necessary for the learning before recording is started, and this hinders the promptness of the operation of the apparatus.
As described above, for example, a recording energy variation that cannot be compensated for by the recording power learning occurs, so that deterioration of recording signal quality cannot be avoided.